Nonvolatile programmable memories operate similar to read only memories with the attribute that they may be programmed at least once. An erasable programmable read-only memory (EPROM) is one type of nonvolatile programmable memory. Typically, the contents of an EPROM are programmed during manufacture, but the user is free to reprogram the contents at a later time.
An EPROM memory cell comprises a single transistor, that when read, is either on "1", or off "0". A transistor is read by applying a small positive electrical charge to the transistor that equals or surpasses the voltage required to read the cell, which is called the threshold voltage V.sub.t. If the transistor is on, then the charge will cause electrons to flow through the transistor, which generates a current representing a 1 bit. If the transistor is off, then no current is generated by the transistor in response to the positive electrical charge, which represents a 0 bit.
Each EPROM transistor includes a structure called a floating-gate. The charge of the floating gate is dependent upon the number of electrons contained in the floating gate. During the programming of an EPROM, the charge of the floating gate for each cell can be altered to control whether each transistor is on or off.
During a procedure called erasing, electrons are pulled out of the floating gate of each cell in the flash EPROM array to neutralize the charge of the floating gate. When a cell is subsequently read, the positive electrical charge applied to the cell is unaffected by the neutrally charged floating gate and current is generated by the cell. Therefore, in the erase state, a cell is "1" or on.
Flash EPROMs suffers a disadvantage compared to EPROMs. EPROMs have the same physical structure as flash EPROM, but are erased differently. An EPROM is erased by exposure to ultraviolet light, which pulls the electrons out of the floating gate. This neutralizes every floating gate in the EPROM so that they have a zero charge.
A flash EPROM, in contrast, is erased by applying an electrical bias to the cells. The bias, however, can deplete the floating gate of electrons, which causes the charge on the floating gate to change from negative to positive, rather than staying neutral. This is referred to as an over-erased cell. When a cell has been electrically erased to depletion mode, its threshold voltage is lowered to the point where the transistor is on even when a zero volts bias is applied to the transistor. In some cases the threshold voltage may even become negative.
Conventional methods to correct the over erased cell are either inadequate in several respects or introduce further problems. One such problem is the over-correction of erased cells in which normally erased cells are changed from "1" (erased) to "0" or on. Over-correction also increases the threshold voltage of cells, which widens the threshold voltage distribution of the flash EPROM and makes the flash EPROM more difficult to read and program.
Accordingly, what is needed is a correction method that provides a flash EPROM that has a uniform threshold voltage distribution during programming.